The Oligocene to the Quaternary: climate and biology - Climate Change: Biological and Human Aspects - page 120

They found growth rates that varied with similar frequency to the present-day El Nino
cycle, suggesting that it was occurring at that time.
The answer to this question is important to ascertain because the early Pliocene
can be considered a palaeoclimatic analogue of a possible late-21st-century globally
warmed Earth. This means that Pliocene research is of interest to those concerned
with present-day climate change.
Antarctica's ice sheets were growing throughout the Pliocene, but were still smaller
than they are today. Similarly, there was ice in the northern hemisphere, but again
this was greatly reduced compared to today. Atmospheric carbon dioxide, although
declining, was also slightly (approximately 30%) higher later in the Pliocene than
recent pre-industrial levels, at around approximately 360 ppm. This compares with
carbon dioxide levels in the 1990s, but the reason that the Pliocene Earth was warmer
than the 1990s is because its oceans had already been warmed, its Pliocene ice
caps were smaller than today (reflecting less sunlight) and there were also some
atmosphere- and ocean-circulation differences.
Associated with this last, tectonic movement of the continents also continued
through the Pliocene and that also impacted on the global climate. The aforementioned
Drake and Tasmanian Passages continued to widen but there was also the closing
of a key tropical gateway linking the Pacific and Atlantic Oceans, with the joining
of North and South America. This affected heat transportation from the tropics as
global ocean circulation re-organised.
In addition to ocean circulation, which is important for heat distribution about the
planet, there were the Milankovitch orbital factors affecting the amount of energy the
Earth receives between hemispheres (before albedo reflection considerations) and at
different times of the year. As stated, the rise of the Tibetan Plateau was central to
this. Evidence from late-Pliocene temperate pollen analysis suggests a temperate-
zone climate periodicity of 124 000 years, with a non-linear response of the climate
system to Milankovitch forcing.
Not only did some temperate ecosystems wax and wane in extent and location due
to climatic changes, but plant populations responded to the new selection pressures.
The rise of C 4 plants was a major evolutionary step, but there were also more mod-
est, but still important, speciations to exploit the cooler Earth. Much Arctic flora is
thought to have originated some 3 mya, towards the end of the Pliocene. The flora
evolved to survive both the cold and the climatic waxing and waning. The likely pat-
tern, as exemplified by Richard Abbott et al. (2000), is that an Arctic species evolved
in one location (different locations for one or more different species) and then spread
in a ring around the pole. As the Pliocene Earth warmed and cooled these species
extended from, and returned to, various ecological refugia. Abbott and his colleagues
demonstrated this by looking at the chloroplast DNA in species of saxifrage (an
order of dicotyledon trees and shrubs), and specifically Saxifraga oppositifolia .As
time passes mutations arise in the DNA and these increase additively with each gen-
eration. Furthermore, if a population becomes increasingly isolated, such as when
confined to small refugia due to adverse climate regimes, the population retains its
characteristic set of mutations. It is therefore possible, through genetic analysis, to
ascertain to which refuge population a plant belongs, and indeed the primary location
where it originally evolved. The evidence of Abbott et al. suggests that in the case of